Back out "Always use first syncmer as base"

This backs out commit 9fa9d41.

Author: marcelm

src/randstrobes.cpp

@@ -49,7 +49,8 @@ static inline syncmer_hash_t syncmer_smer_hash(uint64_t packed) {
  * This function combines two individual syncmer hashes into a single hash
  * for the randstrobe.
  *
- * One syncmer is designated as the "main", the other is the "auxiliary".
+ * The syncmer with the smaller hash is designated as the "main", the other is
+ * the "auxiliary".
  * The combined hash is obtained by setting the top bits to the bits of
  * the main hash and the bottom bits to the bits of the auxiliary
  * hash. Since entries in the index are sorted by randstrobe hash, this allows
@@ -58,6 +59,10 @@ static inline syncmer_hash_t syncmer_smer_hash(uint64_t packed) {
 static inline randstrobe_hash_t randstrobe_hash(
     syncmer_hash_t hash1, syncmer_hash_t hash2, randstrobe_hash_t main_hash_mask
 ) {
+    // Make the function symmetric
+    if (hash1 > hash2) {
+        std::swap(hash1, hash2);
+    }
     return ((hash1 & main_hash_mask) | (hash2 & ~main_hash_mask)) & RANDSTROBE_HASH_MASK;
 }
 
@@ -156,11 +161,12 @@ std::ostream& operator<<(std::ostream& os, const QueryRandstrobe& randstrobe) {
 }
 
 Randstrobe make_randstrobe(Syncmer strobe1, Syncmer strobe2, randstrobe_hash_t main_hash_mask) {
+    bool first_strobe_is_main = strobe1.hash < strobe2.hash;
     return Randstrobe{
         randstrobe_hash(strobe1.hash, strobe2.hash, main_hash_mask),
         static_cast<uint32_t>(strobe1.position),
         static_cast<uint32_t>(strobe2.position),
-        true
+        first_strobe_is_main
     };
 }
 

tests/phix.mcs.se.paf

@@ -1,11 +1,11 @@
-SRR1377138.32	301	2	293	+	NC_001422.1	5386	1434	1725	47	291	255
-SRR1377138.33	301	2	297	+	NC_001422.1	5386	3818	4113	47	295	255
-SRR1377138.34	301	33	299	-	NC_001422.1	5386	844	1110	40	266	255
-SRR1377138.35	301	5	298	-	NC_001422.1	5386	4041	4334	47	293	255
-SRR1377138.36	301	3	301	+	NC_001422.1	5386	4997	5295	51	298	255
-SRR1377138.37	301	2	299	-	NC_001422.1	5386	794	1091	42	297	255
-SRR1377138.38	301	32	284	-	NC_001422.1	5386	4971	5223	42	252	255
-SRR1377138.39/1	301	1	295	-	NC_001422.1	5386	1791	2085	48	294	255
-SRR1377138.40	301	4	293	-	NC_001422.1	5386	3020	3309	44	289	255
-rescuable.42	301	4	293	-	NC_001422.1	5386	3020	3309	44	289	255
-not.rescuable	301	4	293	-	NC_001422.1	5386	3020	3309	44	289	255
+SRR1377138.32	301	2	293	+	NC_001422.1	5386	1434	1725	49	291	255
+SRR1377138.33	301	2	297	+	NC_001422.1	5386	3818	4113	51	295	255
+SRR1377138.34	301	33	299	-	NC_001422.1	5386	844	1110	43	266	255
+SRR1377138.35	301	5	298	-	NC_001422.1	5386	4041	4334	49	293	255
+SRR1377138.36	301	3	301	+	NC_001422.1	5386	4997	5295	53	298	255
+SRR1377138.37	301	2	299	-	NC_001422.1	5386	794	1091	43	297	255
+SRR1377138.38	301	32	284	-	NC_001422.1	5386	4971	5223	44	252	255
+SRR1377138.39/1	301	1	295	-	NC_001422.1	5386	1791	2085	50	294	255
+SRR1377138.40	301	4	293	-	NC_001422.1	5386	3020	3309	48	289	255
+rescuable.42	301	4	293	-	NC_001422.1	5386	3020	3309	48	289	255
+not.rescuable	301	4	293	-	NC_001422.1	5386	3020	3309	48	289	255